Typical state of the art adhesives which are used in bonding friction materials to metal substrates all use phenolic based or n-butyl modified nitrile rubber based thermosetting adhesives. These adhesives contain solvents such as methyl ethyl ketone (MEK), ethanol, methanol, or acetone. The solvent is typically present in an amount ranging from about twenty percent to over eighty percent, by volume, with the remainder being the thermosetting adhesive material. The presence of the solvent allows the adhesive to be thinned out and distributed on the metal substrate in a uniform coating.
These solvent-based adhesives are applied using a variety of methods such as roll coating onto metal or friction paper, flow-coating onto metal or paper, wet spray from a nozzle onto metal, dipping of the metal and the like. The adhesive is applied wet to the substrate (metal or paper) and the solvent is thereafter allowed to evaporate off leaving the adhesive in solid form on the surface. The solvents that evaporate from the adhesive must be captured and reclaimed or burned off using air pollution control equipment in order to meet EPA requirements.
These solvent-based adhesives are considered a health risk with some of the solvent-based adhesives containing carcinogens. Therefore, rigorous air pollution control equipment is required to recover the solvent from the adhesive material.
The solvent-based adhesive is applied to a variety of metal substrates including, for example, transmission bands and core plates, which have irregular shapes. Thus, it has been preferred that the solvent-based adhesive be applied to the metal substrate by dipping the irregularly shaped metal substrate into the solvent-based adhesive. However, this dipping method often causes drips, sags or runs of the adhesive material to occur on the metal substrate. The extra adhesive material in the drip or run must be removed from the substrate prior to the application of the friction material. If the extra adhesive material is not removed, the drip will cause a bump or ridge in the friction material. Any ridge or irregularity is totally unacceptable since its presence will cause the composite friction element to wear unevenly or fail to engage evenly. In addition, it is difficult to control the thickness of the adhesive deposited on the metal substrate. During evaporation of the solvent, the adhesive viscosity changes, which in turn causes the thickness of the adhesive applied to the metal substrate to change. Therefore, process control of adhesive application and adhesive viscosity is difficult to maintain.
Therefore, it is an object of this invention to provide a process for bonding a friction material to a metal substrate without the use of solvent-based adhesives.
A further object of this invention is to provide composite friction elements comprising a metal substrate having adhered thereto without the use of a liquid solvent based adhesive, a friction material.
Yet another object of this invention is to provide a process for bonding a friction material to a metal substrate conveniently and safely without imposing significant threats to human health and the environment by the elimination of the need for liquid solvent based adhesives.